# Water Hardness. Calculating Calcium.

1. Jun 24, 2015

### Dannalee Jayvia

1. The problem statement, all variables and given/known data

I have been asked to determine the concentration of calcium ions, in a 100 mL sample of water.
Eriochrome black T Indicator was used. Magnesium was removed through a precipitate. The water sample changes colour to red. pH 10 buffer solution was also used.
the 3 test amounts of 0.01 mol EDTA solution were
11.07mL, 11.57mL and 10.57mL giving an average titre of 11.07mL

2. Relevant equations
moles(EDTA) = concentration (mol) x volume (L)

Calculate moles of Ca2+ present in the 100.0 mL water sample:
moles Ca2+ = moles EDTA

Calculate concentration (molarity) of calcium ions in the water:
molarity(Ca2+) = moles ÷ volume (L)

This is as far as I could figure out
3. The attempt at a solution
moles(EDTA) = concentration (mol) x volume (L)
= 0.01 x 11.07 x 10-2 = 11.07 x 10-4 mol

Calculate moles of Ca2+ present in the 100.0 mL water sample:
moles Ca2+ = moles EDTA = 11.07 x 10-4 mol

Calculate concentration (molarity) of calcium ions in the water:
molarity(Ca2+) = moles ÷ volume (L) = 11.07 x 10-4 ÷ 100.0 x 10-3

2. Jun 24, 2015

### Staff: Mentor

In general you are on the right track, but your math is incorrect here:

10-2 is wrong (as long as it is intended to be a conversion factor between mL and L, I don't see what else you could use here).